Wiktionary
ring gag
n. (context BDSM English) A gag with a hollow ring that holds the mouth open.
Usage examples of "ring gag".
Zoe had been strapped down onto his dentist's chair and a ring gag forced into her mouth to keep her jaws held open.
Last he replaced the pillowcase with a ring gag that kept her mouth open in an O but made it impossible to speak.